The German supermarket chain Aldi used cameras and detectives to snoop on its employees, finding those who were working slow, had financial problems or were having an affair a former employee has claimed.

In an article published in the German magazine Der Spiegel, the unnamed source, who once worked as a store detective at the discount chain, lifted the lid on an alleged extensive surveillance operation employed by Aldi's German shops to gather information on staff that could be used to the company's advantage.

"I had to report everything that came to my attention. I had to say if an employee was working too slowly, engaged in having an affair and reveal other private details such as their financial situation," the source told the magazine.

He also said his job had been threatened when he objected to installing miniature cameras in the staff changing rooms.

In a statement Aldi denied the allegations, saying it had never given orders to "report 'irregularities' such as private details." The latest allegations levelled against the company followed in the wake of those made by Andreas Straub, a former Aldi manager.

In his book "Simply Cheap", which was published last year, Mr Straub claimed the company blackmailed and coerced staff it wanted rid of, and employed a "system of control and fear."